420 Speedometer Question

Hi All,
I’m converting my 1967 420 sedan from auto trans to 4 speed all synchro overdrive transmission with a compact Laycock unit. I have most of the details sorted, but not sure on the speedometer. The car has a 1020 tpm in it now. It looks like I may need a 1080 tpm speedometer. Does anyone have details on this?

I think it’s the axle ratio which affects the Speedo not the gearbox.
So you’ll be ok.

Thanks! I only ask because on like an mgb the speedometer depends on transmission used.

Hi Jagman66E,

Here is a post by Michael_Frank, This may answer your question.

Nov 3

An important bit of advice for anyone ordering a five speed based on T5 gear sets. Decide on your tire size and pick your speedometer. Then have the builder install the correct speedometer drive gear on the tail shaft (there are many options: 6,7, 8, 9 or 10 teeth). This is a simple choice while the transmission is a-buildin’. Once the box is assembled and in the car, it becomes a big, expensive job to change. If you pick the right drive gear, it’s a snap to recalibrate the speedometer by swapping driven gears in the the transmission pickup. If you pick the wrong one, you will never have an accurate speedometer.

I’ve put up an Excel spreadsheet that will help with the calculation. The green box allows you to vary input data. When it opens, it’s set up for stock tires and a 3.54 rear. To perform the calculation for your own situation, you need the tire width and aspect ratio, differential ratio, and the TPM of the speedometer head. If you want to check the accuracy of your speedometer using an electric drill, you can input the RPM of the drill. You can change the tooth count of the drive gear as well. Finally, you can experiment with % tire wear to see the effect of that. What you want is a drive gear that allows you to pick a driven gear towards the middle of the range…say a blue or white gear. That will allow you to easily recalibrate if you later play with tire size.

http://www.coolcatcorp.com/Speedometer/Speedometer.xls

Illustration of drive and driven gears, along with output housing and retainer:

image

I don’t review this forum, but the ref (thank you) notified me.

Unfortunately, there’s no easy answer to your question. Jaguar transmissions use a speedomter gear housing that’s similar to a T5, and while I’m not sure what’s used with your original BW, the all synchro transmission will have a speedometer bullet that looks something like this:

And this assembly meshes with a helical gear on the tail shaft:

The good news is that these are extremely durable. The bad news is that we’re long past the time when you could go to a Jaguar dealer and buy a new speedometer gear to recalibrate. So what to do?

Get the transmission on the bench and shift it into neutral. Turn the output flange, I think that can be done easily by hand. Observe the speedometer gear and make sure it’s turning. Then turn the output flange exactly ten times, while carefully counting the number of revolutions of the speedometer gear. You will then need to collect the following measurements:

T=circumference of the tire (on rim, fully inflated) in inches. Get this by direct measurement
R=rear axle ratio
S=speedometer gear ratio (calculate by dividing the number of turns of the speedometer gear to turns of the rear flange, as counted earlier)
Then solve for the new TPM of the speedometer head:

TPM= ((63360/T)*R)/S

If the resulting number doesn’t look right, drop me a line with your raw numbers and I’ll check the math. Before we go any further, check the ratio of the angle drive you will be using. Most Jaguar angle drives are 1:1, except for the ones that aren’t. If yours isn’t 1:1, you will have to multiply the TPM by the angle drive ratio. To help your search, the calculation above is usually about 4% high, due to the change in rolling radius when the tires are carrying load.

Now that you know the TPM of the cable, you also know whether or not your speedometer head is ok. Here’s a tip:the internals of your speedometer are interchangeable with most other Smiths of this vintage. So if you can find an instrument with about the right TPM, you can just reset the odometer wheels and swap the guts into your case, calibration is done.

As for the cable, the cable ends should be .120, but examine the angle drive carefully, as some of these used a smaller square. It’s not a problem to have different sized squares on either end, but you need to know that when you order the cable.

Finally, a general point of interest: the threads used on the speedometer cable and angle drive connections are an odd standard called BSB, British Standard Brass. The pitch will be 26 regardless of the diameter. I’m sure there’s a amusing explanation of why this is better, but if you need to rethread any fittings, it’s not going to be funny.

Oops! That should be TPM= ((63360/T)*R)*S

This all seems extremely complicated. When I changed from a 5.12 to 4.55 final drive gears in my MGTD I had Nissonger Instruments of Long Island New York (who are the 60 year long Smiths agents in the USA) re-calibrate my speedo head to match the gearing. Their instructions were to carefully push the car forward on a level surface exactly 52.8 feet forward whilst counting the exact number of revolutions and fractions of the speedo cable when detached from the speedo head. The 52.8 feet being 100th of a mile. Do this three times for accuracy and provide them the number of turns and the head and they re-gear the head. This process takes into account the final drive gearing as well as tyre size. Be sure you are running correct pressures.

If the speedo drive gear is on the gearbox tail shaft the speedo does not care or need to know what that tail shaft is being driven from, auto, manual, four speed, five speed, two speed, doesn’t care, the tyre / wheel size and final drive ratio all matter but not the gearbox!

Thanks I’ll use the 1080 tpm to start with as my parts book indicates. Std rear gearing and tire diameters.